The use of near infrared spectrophotometry in clinical patient examination for medical purposes has grown steadily and increasingly during the last ten to twenty years, and is drawing increasing interest in more recent years, with various new developments, or proposed developments. A very commonly encountered such application is found in pulse oximeters, which have a sensor that is applied to a bodily appendage such as the ear, finger, nasal septurn, etc., to detect the arterial pulse and sample data for determining the oxygen saturation of hemoglobin in the blood supply.
A more recent innovation is the cerebral oxygen saturation monitor developed by Somanetics Corporation of Troy, Mich., U.S.A., which uses sensors that are superficially similar to those used with pulse oximeters but which have distinct differences attributable to their different purpose and different point of application, i.e., the human forehead as opposed to appendages such as ears, fingers, etc., as well as to the different data-sampling and processing techniques which characterize the cerebral oximeter.
Accordingly, examples of the many different sensors intended for pulse oximeters of the type referred to above may be seen in such patents as those to Smart et at., U.S. Pat. No. 3,769,974, Kressee et at., U.S. Pat. No. 4,013,067, New et at., U.S. Pat. Nos. 4,621,643, 4,700,708, 4,770,179, Tan et at., U.S. Pat. No. 4,825,822, Goodman et at., U.S. Pat. No. 4,830,014, Rich et at., U.S. Pat. No. 4,865,038, and Muz, U.S. Pat. No. 5,094,240. There are also other examples of sensors which have certain basic similarities in overall or basic nature and function, as may be seen for example in certain of the different patents to Franz Jobsis, e.g., U.S. Pat. Nos. 4,223,680, 4,281,645, 4,321,930, 4,805,623, 4,380,240 and 4,510,938, although the actual utilization of these has apparently never taken place in any commercial apparatus, and the use and function of the related apparatus is not considered really understood; however, these patents do show electro optical components proposed for use on the human forehead, as well as other portions of the body. Apart from this, sensors which are specifically described as intended for use in cerebral oximeters have apparently only been proposed by inventors associated with Somanetics Corporation, as shown and described for example in U.S. Pat. Nos. 5,139,025 and 5,217,013, and further referred to in pending application Ser. No. 983,820 (filed Dec. 1, 1992).
While significant particular differences exist between probes used in arterial (pulse) oximeters and cerebral oximeters, and the resulting data is processed in a significantly different manner in order to arrive at the desired readout of particular physiologic parameters, the overall structural developments which have taken place in sensors for pulse oximeters is to some extent instructive and informative in connection with cerebral oximeters, even though some of the characteristic problems and considerations encountered in pulse oximeters, where the sensor is in effect wrapped around the finger or the earlobe or the bridge of the nose to position the detector substantially opposite the light source, are not encountered in cerebral oximeters. In addition, the aforementioned Somanetics Corporation cerebral oximeter characteristically uses more than one examining wavelength and both a near and far detect, which are disposed laterally from the source at particular distances which are very important to maintain. Further, while a certain amount of longitudinal flexibility is necessary for a cerebral oximeter sensor to conform to the human forehead, this is of course far less than that required to wrap around the human finger or the like, where the particular lateral distance of the detector from the source is not usually given any consideration, the principal objective being that of maintaining the fight curvature about the finger, etc. and the desired mutually opposed relationship between the electro-optical components. In a cerebral oximeter, the sensor is more nearly flat, since the degree of curvature of the human forehead is much less than that of a finger, etc., and the electro-optical components are disposed laterally adjacent rather than across from one another. Furthermore, the curvature of the forehead is typically compound in nature (i.e., both longitudinal and lateral), whereas the curvature of fingers, etc. is usually considered merely, or generally, cylindrical in nature rather than compound.